In the power electronics field, semiconductor devices, such as insulated gate bipolar transistors (IGBTs) and metal oxide semiconductor field effect transistors (MOSFETs), are widely used as switching elements that control power supply to loads such as motors. Insulated gate semiconductor devices for power control include trench MOSFETs in which gate electrodes are buried in semiconductor layers.
In the conventional trench MOSFET, upon application of high voltage, an electric field is also applied to a silicon oxide film being a gate insulating film. Here, of the silicon oxide film formed in a trench along a trench shape, the highest electric field is applied to the silicon oxide film on a bottom of the trench in which a gate electrode is buried. To relieve the electric field applied to the silicon oxide film on the bottom of the trench, a method for forming a protective diffusion layer of a p-type in a drift layer of a n-type in contact with the bottom of the trench has been developed (for example, see Patent Document 1).
Here, the protective diffusion layer of the p-type is electrically connected to a first base region of the p-type in which a channel region is formed with a second base region of the p-type that is in contact with a trench side wall and is formed in the drift layer of the n-type, whereby a potential is fixed. A trench sidewall surface on which the second base region is formed is in a p-type region, so that MOS characteristics deterionate. In other words, the trench sidewall surface on which the second base region is formed hardly functions as the channel, thereby reducing a density of the channel in the trench MOSFET and increasing on-resistance of the trench MOSFET.